Not only do mulberry trees produce tart, nutritious fruit, but they've also been used for millennia to cultivate silkworms. Their role as ornamental trees in U.S. cities has faced criticism, though, since male mulberry trees release heavily allergenic pollen (pictured).
Habitat: Temperate and subtropical regions of Asia, Africa, Europe and the Americas. Also widely planted in rural and urban areas.
Description: Deciduous, lobe-leafed trees, rarely growing taller than 30 to 50 feet
Allergy season: Spring (April to June)
Tips: Like other dioecious plants, male mulberry trees produce the most pollen. But they can be made less prolific if given a "sex change" by top-grafting them with branches from a female tree. Their pollen may cross-react with other Moraceae species, including fig and breadfruit trees.